This page covers advanced template techniques, in particular the use of variable template names and parameter names in templates. Readers should be thoroughly familiar with the standard template techniques found in Help:Template. Some techniques described below may be outdated or of limited use; for example, ParserFunctions or Lua may be more convenient than some of the branching techniques discussed.

In general, these techniques rely on the recursive (inside-out) processing of templates. When a template is expanded (processed), it translates template code into string values. These are often sent directly to the web browser for display, but they can be treated as code themselves. By enclosing the text strings in double-curly-brackets ( {{}} ), for instance, they can be reprocessed as template names or variable names, producing different results as the string values are changed.

Notes:

This document refers to features currently only available in the Mediawiki software starting with version 1.6 and later.

The term "variable" has two uses in this document:

As noun it means a type of magic word, which is a system-based variable that can be accessed in the same manner as templates (thus {{NAMESPACE}} will return the name of the current namespace, depending on the page).

As adjective or adverb, it is used in the general sense, to indicate that something may vary with context (thus a "variable template name" means that the name of the template being called can change according to parameters or variables).

Using "subst:" with manual recursion gives a stepwise replacement, useful for analyzing and explaining the working of templates calling other templates. For instance, compare {{{{tctc}} }} (discussed below) with {{{{subst:tctc}} }} on a sandbox page. Though both produce the same visible result, the first will remain as the full code {{{{tctc}} }}, while the second will replace the inner template {{tctc}} with its value, leaving the code {{tc }}. See, e.g., Template talk:Lop.

In some cases, it may be useful for a template to call different templates depending on a variable or a passed parameter. This allows a type of branching that can be simpler than with ParserFunctions, though they are generally less versatile.

Examples:

Using a variable to choose a template - {{ {{NAMESPACE}} }}

The magic word {{NAMESPACE}} returns the current namespace, like so: {{NAMESPACE}} = Help. The outer brackets then process this result as a template in its own right - in this case Template:Help - which produces This phrase is the contents of Template:Help.

Using a template to choose a template - {{{{tctc}} }}

the template {{Tctc}} contains the text "tc". This text is processed by the outer brackets as Template:tc which contains the word "in".

The extra spaces in the above examples are needed: without them, the pair of inner three braces is taken as those for a parameter. On the page itself it just shows as plain text: {{{{NAMESPACE}}}}.

Using a parameter to choose a template - {{{{{2}}}x|{{{1}}}}}

The second parameter passed becomes part of the template name to which the first parameter is passed. In this case {{2x|{{{1}}}}} would produce {{{1}}}{{{1}}}, {{3x|{{{1}}}}} would produce {{{1}}}{{{1}}}{{{1}}}, and etc. Template:Hist5 uses this technique repeatedly in {{hist5|X|4|7|3|2|9}} to produce the following 5 line histogram:

Templates can be passed as parameters to other templates. This can mean either that the template is evaluated and the result is passed as a parameter or that the template name is passed and evaluated as part of the other template.

Just as above except {{tc}} (in) is first passed to {{5x}} and the result of that is passed to {{3x}}, to give ininininininininininininininin.

Passing a template name - {{tt|t|V}}

Template:tt - {{{{{1}}}|a{{{2}}}b{{{2}}}c{{{2}}}d}} - takes the value V (passed as the second parameter) and produces aVbVcVd. This value is then passed to template:t (which was passed by name as the first parameter), producing "start-aVbVcVd-end".

A parameter name in a template can be the value of another parameter. This is useful if you want the behavior of a template to change based on the information that is provided to it.

Choosing parameters contextually -{{t pnd|parameter name}}

Template:t p contains {{{capital}}} is the capital of {{{country}}}, with two parameters - "capital" and "country". Template:t pnd containing "{{t p|{{{1}}}=abc}}" can be used to select which parameter is used in a particular case. Thus:

This applies to integer parameters as well, since integer parameters can be specified using the "#=" notation. Passing the integer value N to {{t pnd}} as its parameter will make it look for the Nth unnamed parameter.

Parameter name from another parameter in the same template - {{ppp|p=foo|foo=bar}}

using Template:ppp, which contains {{{{{{p}}}}}}, the code bar will first set the parameter named "foo" to the value "bar", and then set the parameter named "p" to the value of foo, yielding bar. The order in which the parameters appear in the code does not matter, but the technique cannot be applied multiple times—e.g., using Template:tvvv, which contains {{{{{{{{{p}}}}}}}}}, {{tvvv|p=foo|foo=bar|bar=biz}} gives biz.

This is e.g. applied in w:Template:Reg polyhedra db, which contains a 2D array in the form {{{{{1}}}|{{{2}}}|1-1=a11|..|m-n=amn}}. The first parameter is the name of a template that provides a particular selection and presentation of a selected row of the array, e.g. w:Template:Reg polyhedron stat table, the second parameter (which is the first parameter of the latter template) specifies the row. The latter templates references element j of the row concerned by a tag of the form {{{{{{1}}}-j}}}

Whilst the same output could also be produced using {#switch:}, this method is less intensive on the server and may help to stay under page limits; see Help:Array.

The parameter default feature was introduced before Extension:ParserFunctions. This led to the development of branching methods through the parameter default mechanism.

If-defined branches - {{{test{{{test|}}}|{{{then}}}}}}

If no value is passed for the parameter test, then {{{test{{{test|}}}|{{{then}}}}}} resolves to {{{test|{{{then}}}}}} and returns a blank entry (since test is defined as blank). If the parameter "test" is assigned the value "boo", however, {{{test{{{test|}}}|{{{then}}}}}} resolves to {{{testboo|{{{then}}}}}}, and so long as no parameter "testboo" exists, then this will return the value of the parameter "then".

There was also an array technique using parameter defaults, with the disadvantage that a template using this technique had to be called with, in addition to the normal parameters, a standard parameter definition not reflecting a choice, but necessary to make the template work.

An even older branching technique dates from before the introduction of the parameter default mechanism. It is based on the fact that if in a template call a parameter is assigned a value more than once, the last one counts. This is used in combination with specifying the value of a parameter in a template call, where the name of that parameter depends on a parameter of an outer template. In a call {{a|b=c|{{{d}}}=e}}, template:a uses b=c if b≠{{{d}}} and b=e if b={{{d}}}.

Another old "branching technique" is using a template name depending on the value of a parameter (see above).

Template:t curr containing "{{CURRENT{{{1|DAY}}} }}<noinclude>[[Category:Test pages]]</noinclude>" gives the text 3 without parameters, since it defaults to {{CURRENTDAY}}, but {{t curr|DAYNAME}} gives Tuesday while {{t curr|MONTHNAME}} gives March. Any magic word that begins with "CURRENT" can be accessed this way.